In recent years, demand for portable information terminals have been increased as observed in a tablet PC, a smart phone, an electronic book reader, and the like, and in a display surface of these portable devices and in a back surface thereof in some of these devices, a chemically strengthened glass has been used as a cover glass, in which a white or black or another color pigment is printed on the inner side surface of the cover glass (for example, refer to PTL 1 and PTL 2).
The cover glass with a pigment printed on the inner side not only protects the information terminals but also causes the appearance of these devices to look nice and exclusive with glossiness. In addition, by printing a multilayer pigment which has been pattern-treated in an overlapping manner, indication of a brand or logo mark of a product and a window such as a sensor and a surrounding edging pattern are formed on a cover glass.
Such a cover glass is produced, for example, by the following process. First, a glass sheet is cut and the edges are processed to form a blank substrate. Polishing or an etching process may be performed to this blank substrate if necessary. This substrate is further cut into the size of a glass for a cover glass and a necessary perforating process or outer peripheral shape process is performed. Also, the cutting into the size of the cover glass may be performed after a chemical strengthening treatment or printing treatment described below.
A chemical strengthening treatment is executed with respect to the blank substrate or cover glass (hereinafter, also collectively referred to as “glass substrate”) to enhance the strength thereof. Examples of the method for a chemical strengthening treatment include a low temperature type ion exchange method, a high temperature type ion exchange method, a surface crystallization method, and a dealkalization method.
Among these, a low temperature type ion exchange method is frequently executed, in which an alkali ion of a glass surface layer is substituted with a larger ion and a compressive stress is formed on the surface. In general, the glass substrate is immersed into potassium nitrate molten salts having a temperature of 340 to 550° C. so as to exchange a sodium ion (Na+) in the glass with a potassium ion (K+) having a large ion diameter.
As the glass substrate to be subjected to a chemical strengthening treatment, a glass sheet of soda-lime glass or aluminosilicate glass including a great amount of Al2O3, for example, 14% by mass or greater has been used. The aluminosilicate glass is a glass whose composition is designed for a chemical strengthening treatment and the chemical strengthening treatment enables an achievement of the high strength thereof. However, an expensive raw material is used for the production thereof and it is necessary to perform melting at high temperature, accordingly, which takes a cost a lot.
On the other hand, the soda-lime glass is a glass having the same composition as a common window glass and is produced at a low cost. However, it is difficult to obtain the same high strength as the aluminosilicate glass by the chemical strengthening treatment.
In general, the strength properties of the chemically strengthened glass are represented by surface compressive stress (CS; Compressive stress) and depth of compressive stress (DOL; Depth of layer). In the case where a common soda-lime glass is subjected to a chemical strengthening treatment as a blank glass, in general, a chemically strengthened glass having CS of 500 to 600 MPa and DOL of 6 to 10 μm is obtained. In addition, in the case where the aluminosilicate glass is subjected to a chemical strengthening treatment as a blank glass, a chemically strengthened glass having CS of 700 to 850 MPa and DOL of 20 to 100 μm is obtained.
In the glass substrate which has been subjected to a chemical strengthening treatment, subsequently, a desired pigment printing is performed on the single surface thereof. In general, screen printing is used for printing on the glass substrate. The printing is performed such that the glass substrate is provided to a screen printing machine and a pigment ink is applied thereto in a multilayered and overlapping manner so as to form a pattern such as a logo mark.
After printing, heating at a temperature of about 60 to 150° C. is performed in the case of thermosetting pigment ink, and irradiation with ultraviolet rays (UV) is performed to cure the printed pigment ink in the case of a UV curable ink. The printing and curing treatment are repeated in a plurality of times if necessary. The cover glass which has been finished in this way is incorporated into portable devices so that the pigment-printed surface thereof faces the inner side.
Here, the pigment printed on the cover glass may be peeled off in a cleaning step before installing the cover glass to the portable devices or after being used for a certain period of time as the portable devices. The peeling off of pigment printing frequently occurs at the interface portion where the pigment is laminated, the pigment forming a logo mark, a sensor window, an edging pattern, or the like, and defects in the appearance of the devices become remarkable.